The present invention relates to an element for an optical isolator which is used to remove a reflected beam occurring when light emitted from a light source is introduced into various kinds of optical element or an optical fiber. The present invention also relates to an optical isolator and a semiconductor laser module, which employ such an optical isolator element.
Light emitted from a light source, for example, a laser light source, is made incident on a variety of optical elements or an optical fiber, and a part of the incident light is reflected or scattered by the end faces and inside of the optical elements or the optical fiber.
A part of the reflected or scattered light returns to the light source in the form of a reflected beam. To prevent the return of the reflected beam, an optical isolator is employed.
A typical conventional optical isolator employed for this purpose has heretofore been arranged such that two polarizers, each comprising a bulk polarizing beam splitter, are disposed at both sides, respectively, of a flat plate-shaped Faraday rotator, and these three parts are accommodated in a cylindrical magnet. In general, the Faraday rotator is formed with a thickness necessary to rotate the plane of polarization of light with a predetermined wavelength through 45.degree. in a saturated magnetic field, and the two polarizers are adjusted so that the respective directions of transmitted polarization are 45.degree. offset from each other in the direction of rotation.
To obtain a large isolation by employing the optical isolator of the type described above, it has been a conventional practice to use a plurality of optical isolators which are arranged in series.
In the meantime, the optical isolator with the above-described structure comprises a Faraday rotator and two polarizers, which are discrete parts, and therefore needs not only a relatively large number of steps to assemble it but also a troublesome operation to effect an optical adjustment between these parts.
Under these circumstances, there has also been proposed an optical isolator comprising an optical isolator element with a structure in which a flat plate-shaped (multilayer film-shaped) polarizer is bonded directly to each face of a flat plate-shaped Faraday rotator, the optical isolator element being disposed in a cylindrical magnet.
However, the proposed optical isolator, in which a plate-shaped polarizer is bonded to each face of a Faraday rotator to form an integral structure, still suffers from problems listed below:
(1) To secure the optical isolator element in position inside the cylindrical magnet, a given fixing jig is needed, which increases the number of parts required to form the optical isolator, and it is necessary to conduct a troublesome operation for mounting the optical isolator element in the magnet. PA1 (2) If light vertically enters or emerges from the plane of incidence or emergence of the optical isolator element, the reflected light from the end faces of the polarizers and the Faraday rotator returns to a light source, e.g., a semiconductor laser. To prevent the occurrence of such a phenomenon, the optical isolator element should be installed with a predetermined angle so that the planes of incidence and emergence are not perpendicular to the direction of incidence of light. However, in order to secure the optical isolator element with an angle in the cylindrical magnet, it is necessary to employ an element fixing jig with a complicated structure and conduct an even more troublesome operation for mounting the optical isolator element. PA1 (3) Since the Faraday rotation angle of the Faraday rotator varies with temperature or wavelength, the isolation that is effected by an optical isolator system comprising a single optical isolator has a peak at a predetermined temperature or a predetermined wavelength and it is considerably inferior at temperatures or wavelengths other than the predetermined level. Thus, a plurality of optical isolators should be arranged in series to form an optical isolator system in order to obtain a high isolation over a wide temperature or wavelength range. However, if a plurality of optical isolators are installed in series, the number of parts increases and the number of assembling steps also increases, and further the adjustment of optical axes becomes complicated, resulting in low mass-productivity. In addition, since the multi-stage optical isolator system has a long optical path length, the effective aperture (the area where a beam can pass) becomes small. Further, the increase in the optical path length results in an increase in the overall size of the optical isolator system. PA1 (4) In a conventional semiconductor laser array comprising a plurality of laser active regions arrayed in a line in close proximity to each other, an optical isolator of the type described above is disposed at a position corresponding to each laser active region. However, since the reduction in size of each optical isolator is limited, even if the spacing between each pair of adjacent laser active regions can be reduced, the spacing between optical isolators cannot be reduced correspondingly. In other words, it is impossible to achieve a reduction in the overall size of the semiconductor laser array. The limitation on the reduction in size of optical isolators is due to the fact that it is difficult to reduce the size of the cylindrical magnet of each optical isolator and that, if the size of the magnet is reduced, the magnetic force decreases, so that it becomes impossible to apply the necessary magnetic field to the Faraday rotator. In addition, a reduction in the size of the magnet results in a reduction in the effective aperture of each of the optical parts accommodated inside the magnet. PA1 (5) There has heretofore been a semiconductor laser module wherein a coupling lens is disposed on an optical axis at at least one side of the optical isolator and a semiconductor laser is disposed on this optical axis, thereby condensing laser light emitted from the semiconductor laser on an optical fiber, for example, through the coupling lens and the optical isolator. In this type of conventional semiconductor laser module, however, the coupling lens is needed in addition to the optical isolator and hence the number of parts increases and the overall size also increases. In addition, since it is necessary to effect an optical positional adjustment between the coupling lens and the optical axis, the assembly operation of the semiconductor laser module is complicated and takes a great deal of time.